1. Field of the Invention
This invention relates to aircraft and especially an airplane of the tailless, all-wing type.
2. Description of the Related Art
Although aircraft have been undergoing evolutionary changes from their conception almost ninety years ago, airplane design has centered around provision of a central fuselage which supports wing structure and a tail assembly rearwardly from the wing. Conventionally, that tail assembly has been made up of vertical and horizontal stabilizers. Periodically, the wing, which serves as the principal means for supporting the airplane has been supplemented with a forwardly located canard on the fuselage. However, the greatest advances have been in the area of propulsion and particularly the development of turbojet engines.
An airplane moves about three axes through the center of gravity. Thus, means is provided to stabilize and control pitch, yaw, and roll. Static stability means the airplane tends to remain in straight and level flight if properly trimmed until that stability is disturbed. Dynamic stability means that the airplane will tend to return to straight and level flight if displaced by a transitory unbalancing force. Wing dihedral and the tail assembly have been the principal means of providing static and dynamic stability.
Efforts to eliminate the drag producing fuselage and tail surfaces of airplanes have for the most part been unsuccessful primarily because aeronautical engineers were unable to solve the instability and other compromising engineering problems found to be inherent in previously suggested all-wing aircraft designs.
It has long been known that an all-wing airplane would have distinct advantages over conventional types if the stability problems and other engineering compromises could be overcome. An all-wing aircraft has a superior lift to drag ratio because nearly all of the exposed parts create lift. Furthermore, elimination of the fuselage decreases the empty weight of the airplane and loads are distributed over the span of the wing making heavily reinforced structures unnecessary. An all-wing aircraft is able to carry a fixed weight farther and more economically than a conventional airplane having a wing and tail structure secured to the fuselage.
Portions of the fuselage have been used to store cargo and fuel and to house passengers, but relatively large portions of the fuselage still remain hollow and unused.
Recognizing that the fuselage and tail surfaces were responsible for significant drag, design efforts have focused on reduction of parasitic drag created by structures that project outwardly from the aircraft but do not contribute directly to lift. When the assumption is made that an airplane requires only a wing for support in the air, it necessarily follows that the need for the rest of the airplane is questionable and creates parasitic drag.
When efforts were previously made to bring a conventional wing-fuselage-tail assembly airplane into closer conformance to the efficiencies and economies of an all-wing aircraft, the results were for the most part fruitless. For example, increasing the fuel load to gain more range requires an increase in the size of the airplane to carry the additional fuel. A larger engine is then needed to power the bigger airplane. Increased fuel consumption because of the need for a higher horsepower propulsion plant means more fuel must be provided which then requires greater lift, larger surfaces, ad infinitum. There is no substitute for decreasing the empty weight of an airplane and having a much greater lift to drag ratio.
The advantages of all-wing airplane designs have long been known and proposals have been made to eliminate the tail surfaces and fuselage structure of airplanes but with little success. One design that came the closest is illustrated and described in U.S. Pat. No. 2,406,506 to John K. Northrop of Northrop Aircraft, Inc. The Northrop "Flying Wing" which was actually built and tested beginning in the 1940s, never became a commercial design. Some of the reasons were stability difficulties and limitations that the elevons imposed on the maximum coefficient of lift at take-off and landing.
The Northrop wing employed a near-symmetrical airfoil incorporating reflex camber at the trailing edge and no dihedral. Control of the Northrop wing in flight was accomplished by providing movable sections along the trailing edge of the wing. Two trim sections were provided with each being located near a respective wing tip. Larger sections inboard of the trim flaps served as a combination aileron and elevator denominated an "elevon". The trim flaps had a combination pitch and roll trim function. The rudders acted as drag surfaces to create a yawing moment rather than a conventional deflection-type rudder. Each rudder was hinged to an electrically-operated trim flap. The rudders moved with the trim flaps for trim purposes but operated independently for yaw control. Foot pressure on one of the rudder pedals opened a corresponding rudder; its surfaces split deflecting above and below the trim flaps.
Lack of success of the Northrop flying wing design can be attributed to a number of most obvious reasons. They included: complaint by pilots of the lack of inherent stability making it impractical to fly the airplane hands-off; use of the elevons negatively affected the maximum coefficient of lift; the lever arm of the elevons was too short in relation to the center of gravity; a download was required on the elevons and trim flaps in an effort to maintain pitch stabilization; the elevons and trim flaps were unable to trim out the negative pitching moment caused by high speed compressibility center of lift shift in an aft direction; nearly ineffective flaps; engine placement requiring extension drives to maintain the empty center of gravity within the center of gravity envelope in the case of the propeller-driven configuration; and lack of static yaw stability.
An especially significant drawback of the Northrop all-wing aircraft was loss of maximum lift coefficient at take-off and landing. The elevons, made up of movable segments of the wing's trailing edge and hinged to act as a combination aileron and elevator, provided pitch control and pitch trim. The movement of this surface changed the camber of the attached airfoil and therefore affected wing lift, drag, and pitching moment. When these surfaces moved up, it reduced wing camber and therefore wing lift, reducing the wing's maximum lift coefficient by 20 to 40%, just when the most lift was needed, at lift-off and landing. The results were take-off, stall and landing speeds considerably higher than with a conventional airplane.
Another significant drawback was that the lever arm for each of the elevons providing pitch control action were too short. This meant that the wing's center of gravity was maintained forward of the center of lift. As a result, a negative pitching moment was generated which in turn had to be counterbalanced by deflection of the elevons upwardly, creating a download to control and trim the airplane about the pitch axis. The short distance of the elevons in relation to the center of gravity and their small surface required them to generate a significant down force to stabilize the airplane about the pitch axis, thus negating a well established proposition that all surfaces of prior art flying wings generated lift. In reality, the Northrop elevons created considerable down force.
A further significant drawback of the Northrop elevon and trim flap arrangement was inability to trim out the negative pitching moment caused by high speed compressibility center of lift shift in an aft direction. The shock waves that developed at high air speeds and high altitudes during operation of the Northrop wing caused the center of lift to move aft on the wing, increasing the distance from the center of gravity to the center of lift, and generating an even greater down pitching moment that eventually overpowered the trim flaps and elevons and placed the airplane in an uncontrollable nose down attitude. The elevons were proposed as a means to meet the combined requirements that the airplane be both statically and dynamically stable and in trim throughout the entire flight envelope. In fact, the many negative qualities of the elevons demonstrated that they were incapable of meeting these requirements.
In addition, the engine placement of the Northrop flying wing required extension drive lines to maintain the empty center of gravity in required deposition in the case of a propeller-driven configuration. The configuration of the flying wing did not permit unobstructed placement of engines forward on the wing to establish an empty forward center of gravity without the use of extension drive shafts for the propellers. Even when pure jets were substituted for a propeller-driven design, the jet engine installation required extended intake ducting with accompanying inlet friction drag.
The hinge points of the inboard flaps of the Northrop flying wing were in close proximity to the hinge point of the elevons. The result was a near self-defeating condition. As the flaps were deployed, a strong nose down pitching moment was induced. To offset this, the elevons and trim flaps were automatically trimmed up. Because of the relative hinge points and relations to the center of gravity, the net result was near zero for increased lift coefficients and a large increase in drag.
The combination pitch and roll trim flaps near the tips of the Northrop wing were also drag producing devices by virtue of the fact that they split, deflecting above and below the trim flaps, creating yawing forces but did not provide for static yaw stability. As a consequence, the wing tended to stray from straight flight when the wing was subjected to the slightest unbalancing force. Northrop's wing had very little static stability about the yaw axis. Sweep back of the leading edge of the wing did provide some weather-cock stability but not enough for satisfactory handling characteristics.
It therefore necessarily follows that the Northrop wing was inherently unstable and not capable of fully stabilized flight. At that point in time, stability augmentation computer technology was not available for operating the control surfaces of an airplane which is inherently unstable. Such techniques do exist today but are expensive, they are not without operating flaws, the systems are unable to control an aircraft once it is outside of its normal flight envelope, and the techniques involve design limitations which restrict the overall utility of the airplane. An airplane that has inherent aerodynamic stability and has a performance envelope just equal to a computer control airplane would still be superior by virtue of the fact that there are no electronic complexities to potentially fail.
So-called V/STOL "lifting body" aircraft are shown and described in Aereon Corporation U.S. Pat. Nos. 3,761,041 and 4,149,688, but these are not all-wing airplanes comparable to the Northrop Flying Wing, or the all-wing airplane of this invention, in that they are essentially triangular in form and do not have stability augmenting surfaces as now provided.
Baume in U.S. Pat. No. 1,977,843 illustrates a flying machine somewhat symbolic of the Arabian Nights Flying Carpet but there is no evidence that this design was ever commercialized or found to have satisfactory flying characteristics. In like manner, the tailless flying wings of Charpenter in U.S. Pat. Nos. 1,893,129 and 2,123,096 were said to incorporate air foils for improved air flow over the wing surface, but control stability was not specifically addressed.
The Delta wing airplane of U.S. Pat. No. 3,625,459, which is somewhat reminiscent of a flying bat, suggested that directional control could be provided by canted wing tip "rudderizers". No assertion was made that the proposed design would be inherently stable.
Other patents disclosing various types of non-conventional flying surface structures include U.S. Pat. Nos. 4,019,699, 1,880,520, 2,937,827, 2,670,155, 1,825,578, and 3,216,673.